Since all packages are global in scope, nesting of packages is not supported. However, you can have two packages, one called
A
and another called
A::B
, to give an illusion of nesting. This is a naming convention only and implies no necessary relation between the two packages; however, this convention is typically applied to groups of related packages, and the term "nested packages" does not seem wrong in such cases. For example, you could have a module called Math::Matrix for matrix manipulation and another called Math::Poisson that supports an infrastructure for simulating queuing models. The only relation between the two modules is that they both are mathematical in nature; they don't share any implementation characteristics.

The
::
notation is used as before to access variables and subroutines of nested packages:

When you say
use
File
, recall that Perl looks for a file called
File.pm
. If you say,
use
Math::Poisson
, Perl looks for a file called
Math/Poisson.pm
(directory
Math
, file
Poisson.pm
). The double colon gets translated to a filename separator, because the colon has a special significance for DOS filenames. Perl imposes no limits on the level of nesting.